LM2931
Application Hints
One of the distinguishing factors of the LM2931 series regu-
lators is the requirement of an output capacitor for device
stability. The value required varies greatly depending upon
the application circuit and other factors. Thus some com-
ments on the characteristics of both capacitors and the reg-
ulator are in order.
High frequency characteristics of electrolytic capacitors de-
pend greatly on the type and even the manufacturer. As a
result, a value of capacitance that works well with the LM2931
for one brand or type may not necessary be sufficient with an
electrolytic of different origin. Sometimes actual bench test-
ing, as described later, will be the only means to determine
the proper capacitor type and value. Experience has shown
that, as a rule of thumb, the more expensive and higher quality
electrolytics generally allow a smaller value for regulator sta-
bility. As an example, while a high-quality 100
μF
aluminum
electrolytic covers all general application circuits, similar sta-
bility can be obtained with a tantalum electrolytic of only
47μF. This factor of two can generally be applied to any spe-
cial application circuit also.
Another critical characteristic of electrolytics is their perfor-
mance over temperature. While the LM2931 is designed to
operate to −40°C, the same is not always true with all elec-
trolytics (hot is generally not a problem). The electrolyte in
many aluminum types will freeze around −30°C, reducing
their effective value to zero. Since the capacitance is needed
for regulator stability, the natural result is oscillation (and lots
of it) at the regulator output. For all application circuits where
cold operation is necessary, the output capacitor must be rat-
ed to operate at the minimum temperature. By coincidence,
worst-case stability for the LM2931 also occurs at minimum
temperatures. As a result, in applications where the regulator
junction temperature will never be less than 25°C, the output
capacitor can be reduced approximately by a factor of two
over the value needed for the entire temperature range. To
continue our example with the tantalum electrolytic, a value
of only 22μF would probably thus suffice. For high-quality alu-
minum, 47μF would be adequate in such an application.
Another regulator characteristic that is noteworthy is that sta-
bility decreases with higher output currents. This sensible fact
has important connotations. In many applications, the
LM2931 is operated at only a few milliamps of output current
or less. In such a circuit, the output capacitor can be further
reduced in value. As a rough estimation, a circuit that is re-
quired to deliver a maximum of 10mA of output current from
the regulator would need an output capacitor of only half the
value compared to the same regulator required to deliver the
full output current of 100mA. If the example of the tantalum
capacitor in the circuit rated at 25°C junction temperature and
above were continued to include a maximum of 10mA of out-
put current, then the 22μF output capacitor could be reduced
to only 10μF.
In the case of the LM2931CT adjustable regulator, the mini-
mum value of output capacitance is a function of the output
voltage. As a general rule, the value decreases with higher
output voltages, since internal loop gain is reduced.
At this point, the procedure for bench testing the minimum
value of an output capacitor in a special application circuit
should be clear. Since worst-case occurs at minimum oper-
ating temperatures and maximum operating currents, the
entire circuit, including the electrolytic, should be cooled to the
minimum temperature. The input voltage to the regulator
should be maintained at 0.6V above the output to keep inter-
nal power dissipation and die heating to a minimum. Worst-
case occurs just after input power is applied and before the
die has had a chance to heat up. Once the minimum value of
capacitance has been found for the brand and type of elec-
trolytic in question, the value should be doubled for actual use
to account for production variations both in the capacitor and
the regulator. (All the values in this section and the remainder
of the data sheet were determined in this fashion.)
LM2931 micro SMD Light Sensitivity
When the LM2931 micro SMD package is exposed to bright
sunlight, normal office fluorescent light, and other LED's, it
operates within the guaranteed limits specified in the electri-
cal characteristic table.
Definition of Terms
Dropout Voltage:
The input-output voltage differential at
which the circuit ceases to regulate against further reduction
in input voltage. Measured when the output voltage has
dropped 100 mV from the nominal value obtained at 14V in-
put, dropout voltage is dependent upon load current and
junction temperature.
Input Voltage:
The DC voltage applied to the input terminals
with respect to ground.
Input-Output Differential:
The voltage difference between
the unregulated input voltage and the regulated output volt-
age for which the regulator will operate.
Line Regulation:
The change in output voltage for a change
in the input voltage. The measurement is made under condi-
tions of low dissipation or by using pulse techniques such that
the average chip temperature is not significantly affected.
Load Regulation:
The change in output voltage for a change
in load current at constant chip temperature.
Long Term Stability:
Output voltage stability under accel-
erated life-test conditions after 1000 hours with maximum
rated voltage and junction temperature.
Output Noise Voltage:
The rms AC voltage at the output,
with constant load and no input ripple, measured over a spec-
ified frequency range.
Quiescent Current:
That part of the positive input current
that does not contribute to the positive load current. The reg-
ulator ground lead current.
Ripple Rejection:
The ratio of the peak-to-peak input ripple
voltage to the peak-to-peak output ripple voltage at a speci-
fied frequency.
Temperature Stability of V
O
:
The percentage change in
output voltage for a thermal variation from room temperature
to either temperature extreme.
13
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